The burning of wood chips and other combustible chip-like materials is desirable or even necessary, for example as an alternative fuel or in the production of heat. It is also desirable to burn chip-like material to dispose of waste from industrial processes, such as from a paper mill, in a manner that recovers energy.
Unfortunately, the chips produced from recently harvested trees usually contain from 40% to 50% moisture. While chips containing this much moisture can be burned, chips dried to have a 10% to 15% water content can be burned with higher efficiency, and with significant increase in the capacity of a boiler plant.
The technology peculiar to chip drying has heretofore not adequately addressed the needs of many applications. Consequently, many plants still burn green chips, and much combustible waste which can be in chip form is not recovered.
However, it has been demonstrated that chips can be dried efficiently in a dryer using waste heat from flue gas or heat from another drying medium. The use of such a dryer can substantially reduce boiler plant operating costs, both by reducing the fuel requirement and by improving plant output. In designing a dryer for chip-like combustible material, several factors stand out. These include thermal efficiency and temperature control, gas flow, material handling, and use of space. The treatment of such factors governs the rate and efficiency, and overall suitability, of the drying.
The selection of the operating temperature, and the realizable thermal efficiency of the dryer at that temperature, depend on the nature of the material to be dried. In drying materials such as wood chips, in which moisture is locked within the body of the chip and is evaporated from the chip's surface during drying, the ratio of chip surface area to volume is important.
Also important is the rate at which a chip can be dried, and this depends at least in part on gas flow and temperature. However, increasing the temperature to improve the drying speed has limits, imposed by the need to avoid pyrolysis of the chips, which for wood chips can commence at temperatures as low as 400.degree. F. to 450.degree. F.
Increasing flow velocity of the heating medium also has limits, imposed for example by the power required to move the gas, and the desirability of avoiding fluidizing solids in the dryer. An often more stringent constraint on gas flow velocity is the desirability of avoiding particulate entrainment, i.e., the carrying away of small particles. For many applications, particulate entrainment in the exhaust is unacceptable by air quality standards, and necessitates subsequent air cleaning.
Several issued patents aid in placing the present invention in context.
U.S. Pat. No. 4,530,700 of Sawyer discloses apparatus for preparing wood chips and other bio-mass material at the harvesting site for consumption as a fuel or chemical feed stock. The apparatus includes a dryer section and a gas producing section. The gas producing section consumes green chips and delivers hot combustion gases to the drying section for drying other green chips.
In the earlier U.S. Pat. No. 4,258,476 of Caughey, a dryer for particulate material such as sawdust has a generally vertical, stack-like drying chamber defined by louvered baffle plates which are reciprocated in advancing the sawdust. In normal operation, the dryer is maintained full of sawdust. The relatively tall column of material in the drying chamber is in the path of flowing air.
In U.S. Pat. No. 4,371,375 of Dennis, an apparatus for drying sawdust and wood chips has an upright housing with a plurality of vertically-spaced, horizontally-disposed dryer plates. Each plate has spaced-apart openings through which sawdust introduced at the top of the housing may pass in sinuous fashion. As warm air flows upwardly in the housing, the falling sawdust is agitated on each plate.
An object of this invention is to provide an improved dryer for chip-like material.
A further object of the invention is to provide a dryer with improved thermal efficiency and temperature control, while avoiding pyrolysis of the dried material.
Yet a further object of the invention is to provide a dryer with improved gas flow characteristics and gas handling power requirements, and reduced particulate entrainment.
A further object of the invention is to provide a dryer with improved material handling, and which provides increased capacity and improves space utilization, preferably through a modular design.